Phytochrome B (PHYB) is one of a family of red/far-red photoreceptors involved in reading the spectral quality of a plant's light environment. PHYB both independently and along with the other phytochromes affects many aspects of development throughout a plant's life cycle from seed germination to floral induction. The importance of these photoreceptors is underlined by the 50+ years of extensive and excellent research surrounding these processes. Mutations in PHYB (called hy3 or phyB), the major phytochrome affecting development throughout the Arabidopsis life cycle, have been identified. l will isolate additional weak alleles of phyB with an emphasis on those caused by missense mutations. photobiological analysis of one missense allele, phyB-4, will address the photoconversion properties of this pigment. I have begun to isolate suppressors of phyB-4. l am proposing to use genetic, morphological and physiological analysis to characterize these suppressor mutants and identify those which are likely to interact with PHYB in an allele or pathway specific manner. Based on this analysis, l will initiate a chromosome walk and clone a gene which is likely to be involved in an early step in PHYB signal transduction. This information will contribute to the general knowledge of how plants use light as a source of information, tailoring their growth patterns for changing environmental cues.